Apparatus And Method For Separating Air By Cryogenic Distillation

ABSTRACT

An apparatus and to a method for producing krypton and xenon by separating air by cryogenic distillation and in particular to a method for separating gasses from air using internal compression (in which the gaseous oxygen is produced by vaporization under pressure in the exchange line) where a krypton/xenon mixture is extracted is provided.

The present invention relates to an apparatus and to a method forproducing krypton and xenon by separating air by cryogenic distillationand in particular to a method for separating gasses from air usinginternal compression (in which the gaseous oxygen is produced byvaporization under pressure in the exchange line) where a krypton/xenonmixture is extracted.

In a unit for separating gasses from air comprising a double column andwhere the gaseous oxygen is produced by vaporization under pressure inthe exchange line (internal compression apparatus) in which provision ismade to extract a krypton/xenon mixture, it is observed that theefficiency of this extraction decreases significantly because the pumpedoxygen extracted from the low pressure distillation column brings withit a non-negligible proportion of components to be valorized, such askrypton and xenon.

For this type of internal compression apparatus, in the case of theproduction of krypton/xenon associated with a production of argon,several methods making it possible to overcome this problem have beenconsidered.

The invention can summarized as two essential steps:

-   -   1. Impoverishment of the gaseous air entering the column for        separation into components to be valorized (krypton, xenon) by        transfer of material with a reduced proportion of the rich        liquid. This step gives a lower rich liquid, rich in krypton and        xenon, preferably with 100% of the xenon thus contained in the        gaseous air being extracted as well as more than 80% of the        krypton.    -   2. Concentration of the lower rich liquid rich in krypton and in        xenon, by partial vaporization in a vaporizer/condenser. The        heat source used for vaporizing the lower rich liquid is        preferably a fluid of the column from which comes said lower        rich liquid, a fluid coming from a higher distillation stage        which, on giving up its heat, condenses in the        vaporizer/condenser. The liquid thus produced feeds the low        pressure column at an appropriate distillation level. The lower        rich liquid itself produces a vaporized phase which feeds the        low pressure column at an appropriate distillation level, and a        purge where preferably approximately 100% of the xenon contained        in the feed is extracted, whereas only about 50% of the krypton        is present there.

According to a variant, the low pressure column is fed by the purge richin krypton and in xenon produced by concentration. The purge rich inkrypton and in xenon produced above will feed the low pressure column,at a distillation stage lower than the tapping of the liquid oxygenwhich is intended to be pumped in order to then be vaporized in the mainexchanger. The final mixture, rich in krypton and in xenon, is extractedfrom the bottom of the low pressure column in order to be subsequentlytreated there.

Optionally, in the case where the krypton and xenon mixture is in liquidform, this return is injected at the intake of the pump in order tominimize the OL tapping from the column and thus to increase theextraction yield of the krypton and xenon.

Such a method makes it possible to extract about 90% of the xenonpresent in the air where a conventional method only allows a yield ofabout 70% at best. With regard to the extraction of the krypton, thisreaches about 75% whereas the yield of a conventional apparatus at bestonly allows a yield of the order of 60%.

According to one purpose of the invention, there is provided a methodfor producing krypton and xenon by separation of air by cryogenicdistillation in a double column comprising a medium pressure column anda low pressure column in which:

-   -   i) cooled and purified compressed gaseous air is sent to a first        level of the medium pressure column    -   ii) cooled and purified compressed liquid air is sent to a        second level of the medium pressure column, above the first        level    -   iii) a liquid, enriched in oxygen, krypton and xenon is tapped        from the bottom of the medium pressure column    -   iv) a liquid is tapped from the medium pressure column, at a        level between the first and second levels, and sent to the low        pressure column    -   v) the liquid enriched in oxygen, krypton and xenon is partially        vaporized in order to form an intermediate gas and an        intermediate liquid    -   vi) a first liquid stream rich in oxygen is tapped from a third        level of the low pressure column    -   vii) the intermediate liquid is sent from the vaporizer to the        low pressure column at an intermediate level of the low pressure        column, lower than the third level, or it is mixed with a second        liquid stream rich in oxygen    -   viii) a/the second oxygen-rich liquid stream is tapped from the        low pressure column at a fourth level, lower than the third        level, and, if applicable, at the intermediate level.

According to optional features:

-   -   the intermediate gas is sent from the vaporizer to the low        pressure column at a level higher than the intermediate level;    -   the liquid enriched in oxygen, krypton and xenon is vaporized by        heat exchange with a gas tapped from the medium pressure column;    -   the gas tapped from the medium pressure column is tapped from        above the first level;    -   the intermediate liquid is sent from the vaporizer to an        intermediate level of the low pressure column, lower than the        third level;    -   the second oxygen-rich liquid stream (11) is tapped from the low        pressure column at a fourth level, lower than the intermediate        level;    -   the intermediate liquid is mixed with a second oxygen-rich        liquid stream.

According to another purpose of the invention, there is provided anapparatus for producing krypton and xenon by separation of air bycryogenic distillation comprising a double column constituted by amedium pressure column and a low pressure column and a vaporizercomprising:

-   -   i) means for sending cooled and purified compressed gaseous air        to a first level of the medium pressure column    -   ii) means for sending cooled and purified compressed liquid air        to a second level of the medium pressure column, above the first        level    -   iii) means for tapping a liquid, enriched in oxygen, krypton and        xenon from the bottom of the medium pressure column    -   iv) means for tapping a liquid from the medium pressure column,        at a level between the first and second levels, and it is sent        to the low pressure column    -   v) means for sending at least a portion of the liquid enriched        in oxygen, krypton and xenon to the vaporizer where it is        partially vaporized to form an intermediate gas and an        intermediate liquid    -   vi) means for tapping a first liquid stream rich in oxygen from        a third level of the low pressure column    -   vii) means for sending the intermediate liquid from the        vaporizer to the low pressure column at an intermediate level of        the low pressure column or for mixing it with a second liquid        stream rich in oxygen and    -   viii) means for tapping a/the second oxygen-rich liquid stream        from the low pressure column at a fourth level, lower than the        third level, and, if applicable, at the intermediate level.

The apparatus possibly comprises:

-   -   means for sending the intermediate gas from the vaporizer to the        low pressure column at a level higher than the intermediate        level;    -   means for sending a gas tapped from the medium pressure column        to the vaporizer;    -   means for tapping the gas from the medium pressure column is        tapped from above the first level;    -   means for pressurizing and vaporizing the first oxygen-rich        stream, preferably by heat exchange with the air to be distilled    -   means for sending the intermediate liquid from the vaporizer to        an intermediate level of the low pressure column, lower than the        third level;    -   means for tapping the second oxygen-rich liquid (11) from the        low pressure column at a fourth level, lower than the        intermediate level    -   means for mixing the intermediate liquid with a second        oxygen-rich liquid stream.

The invention will be described in more detail with reference to theFIGURE.

A stream of medium-pressure gaseous air 1 is sent to the medium pressurecolumn 20 of a double air separation column. The double column alsocomprises a low pressure column 21 thermally coupled with the mediumpressure column 20 by means of a vaporizer 24, which vaporizes thebottom liquid of the low pressure column by heat exchange with thegaseous nitrogen at the top of the medium pressure column.

A high-pressure liquid air stream is expanded in a valve and thendivided into two. The stream 3 thus formed is sent to the mediumpressure column 20 a few theoretical trays above the entry point of thestream 1. The rest of the liquid air 4 is cooled in the subcooler 26,expanded and then sent to the low pressure column 21.

A lower stream of rich liquid 5, enriched in oxygen, krypton and xenon,is tapped from the bottom of the medium pressure column 20, cooled inthe subcooler 26 and then expanded and sent to a vaporizer 22 where itpartially vaporizes by heat exchange with the nitrogen-enriched gaseousair 8 coming from the medium pressure column 20. The gaseous air iscondensed in order to form a stream 9 which is then sent to the lowpressure column 21. The vaporized rich liquid 16 is expanded as anintermediate gas and sent to the low pressure column whilst theremaining intermediate liquid 7 is sent to the low pressure column 21 afew trays above the bottom. A higher stream of rich liquid 6 is tappedbetween the air intake 1 and the air intake 3 and/or the tapping of thestream 8. This stream 6 is cooled in the subcooler 26, mixed with thevaporized rich liquid 16 and sent to the low pressure column 21.Impoverished liquid, enriched with nitrogen 15, is tapped at the top ofthe medium pressure column 20 and sent to the top of the low pressurecolumn 21 after cooling and expansion.

Liquid oxygen 10 is tapped from the low pressure column 21 a few traysabove the bottom and preferably above the liquid intake 7. This liquidis pumped by the pump 23 in order to serve as an oxygen-rich product.

Liquid oxygen 11, enriched with krypton and xenon, is tapped from thebottom of the low pressure column 21 and can be purified subsequently inorder to produce a mixture of krypton and xenon. The intermediate liquid7 can be mixed directly with this liquid oxygen stream instead of beingsent to the low pressure column 21.

Low pressure nitrogen 13 is tapped at the top of the low pressure column21, heated in the subcooler and heated against the air streams 1, 2(heating not shown).

Items of equipment:20 Medium pressure column, 21 Low pressure column, 22 Lower rich liquidvaporizer, 23 Liquid oxygen pump.

Fluids:

1 Medium pressure gaseous air, 2 High pressure liquid air, 3 Expandedliquid air feeding the medium pressure column, 4 Expanded liquid airfeeding the low pressure column, 5 Lower rich liquid, enriched inkrypton and xenon, 6 Vaporized lower rich liquid, impoverished inkrypton and xenon, 7 Purge rich in krypton and xenon, 8 Impoverished airextracted from the medium pressure column, 9 Liquid impoverished air, 10Liquid oxygen extracted from the low pressure column, 11 Liquid oxygenextracted from the low pressure column rich in krypton and xenon.

1-15. (canceled)
 16. A method for producing krypton and xenon byseparation of air by cryogenic distillation in a double columncomprising a medium pressure column and a low pressure column, saidmethod comprising: i) sending cooled and purified compressed gaseous airto a first level of the medium pressure column; ii) sending cooled andpurified compressed liquid air to a second level of the medium pressurecolumn, wherein said second level is above the first level; iii)withdrawing a liquid, enriched in oxygen, krypton and xenon from thebottom of the medium pressure column; iv) withdrawing a liquid from themedium pressure column, at a level between the first and second levels,and sent to the low pressure column; v) partially vaporizing the liquidenriched in oxygen, krypton and xenon in order to form an intermediategas and an intermediate liquid; vi) withdrawing a first liquid streamrich in oxygen from a third level of the low pressure column; vii)sending the intermediate liquid from the vaporizer to the low pressurecolumn at an intermediate level of the low pressure column, lower thanthe third level, and viii) a second oxygen-rich liquid stream is tappedfrom the low pressure column at a fourth level, lower than the thirdlevel, and, lower than the intermediate level.
 17. The method of claim16, wherein the vaporized intermediate gas is sent to the low pressurecolumn at a level higher than the intermediate level.
 18. The method ofclaim 16, wherein the liquid enriched in oxygen, krypton and xenon isvaporized by heat exchange with a gas tapped from the medium pressurecolumn.
 19. The method of claim 18, wherein the gas tapped from themedium pressure column is tapped from above the first level.
 20. Themethod of claim 16, wherein the intermediate liquid is sent from thevaporizer to an intermediate level of the low pressure column, lowerthan the third level.
 21. The method of claim 20, wherein the secondoxygen-rich liquid stream is withdrawn from the low pressure column at afourth level, lower than the intermediate level.
 22. The method of claim16, wherein the intermediate liquid is mixed with a second oxygen-richliquid stream.
 23. An apparatus for separation of air by cryogenicdistillation comprising a double column constituted by a medium pressurecolumn and a low pressure column and a vaporizer comprising: i) meansfor sending cooled and purified compressed gaseous air to a first levelof the medium pressure column ii) means for sending cooled and purifiedcompressed liquid air to a second level of the medium pressure column,above the first level iii) means for tapping a liquid, enriched inoxygen, krypton and xenon from the bottom of the medium pressure columniv) means for tapping a liquid from the medium pressure column, at alevel between the first and second levels, and it is sent to the lowpressure column v) means for sending at least a portion of the liquidenriched in oxygen, krypton and xenon to the vaporizer where it ispartially vaporized to form an intermediate gas and an intermediateliquid vi) means for tapping a first liquid stream rich in oxygen from athird level of the low pressure column vii) means for sending theintermediate liquid from the vaporizer to the low pressure column at anintermediate level of the low pressure column or for mixing it with asecond liquid stream rich in oxygen and viii) means for tapping a/thesecond oxygen-rich liquid stream from the low pressure column at afourth level, lower than the third level, and lower than theintermediate level.
 24. The apparatus of claim 23, further comprisingmeans for sending the intermediate gas from the vaporizer to the lowpressure column at a level higher than the intermediate level.
 25. Theapparatus of claim 23, further comprising means for sending a gas tappedfrom the medium pressure column to the vaporizer.
 26. The apparatus ofclaim 25, further comprising means for tapping the gas from the mediumpressure column from above the first level.
 27. The apparatus of claim23, further comprising means for pressurizing and vaporizing the firstoxygen-rich stream,
 28. The apparatus of claim 27, wherein said meansfor pressurizing and vaporizing the first oxygen-rich stream is by heatexchange with the air to be distilled.
 29. The apparatus of claim 23,further comprising means for sending the intermediate liquid from thevaporizer to an intermediate level of the low pressure column, lowerthan the third level.
 30. The apparatus of claim 29, further comprisingmeans for tapping the second oxygen-rich liquid stream from the lowpressure column at a fourth level, lower than the intermediate level.31. The apparatus of claim 23, further comprising means for mixing theintermediate liquid with a second oxygen-rich liquid stream.